Pressurized containers are widely used for a number of purposes such as: storage of gases such as oxygen, nitrogen, natural gas and propane; pressure packaging and dispensing consumer products such as paints, lacquers, varnishes, food products, hair spray, deodorants, shaving lather, insecticides and herbicides; and packaging for electrochemical cells. The pressurized containers used for pressure packaging and dispensing consumer products are typically aerosol containers which contain a consumer product which is mixed with a propellant gas such as freon or methyl chloride.
Pressurized containers are potentially dangerous because an explosion can result upon overpressurization. Overpressurization can result when a container is overfilled. More frequently, however, overpressurization occurs when the container and its contents are subjected to elevated temperatures during incineration or by storage at unacceptably high temperatures. Overpressurization can also occur as the result of unwanted chemical reactions taking place within the container. This situation can occur in a sealed storage battery which releases gases internally upon overcharge or overdischarge. To provide a safety measure, lithium batteries are enclosed in a casing which contains a pressure release vent. The vent releases on overpressure and prevents any possibility of accidental explosion. Many lithium batteries utilize sulfur dioxide as an electrolyte component. Such a battery desirably has a vent that releases at pressure above about 350 psi.
Willis U.S. Pat. No. 3,918,610, Nov. 11, 1975, discloses a safety vent for a pressurized container which comprises an integral concavity in the container wall, an integral hollow bridge interrupting the concavity, and a weakening score line in the container wall extending across the hollow bridge. When excessive pressure builds up in the container, it acts to stress the bridge. This results in a fracture of the residual container wall under the weakening score line. The pressurized contents vent through the fracture. The approach set forth by Willis is not entirely satisfactory because consistent quality control is difficult to achieve. In the Willis vent, the wall thickness under the score line is a critical parameter if operation is to reliably take place at a predetermined pressure range. Thickness tolerances for proper venting are therefore undesirably small. With the Willis vent, during the manufacturing process, the score is put into the container while it is flat. Subsequently, as concavities and bridges are formed in the container, the score becomes stretched by the forming process.
A. Romero U.S. Pat. No. 4,601,959, July 22, 1986, discloses a metal casing for a pressurized container which is hermetically sealed and has a thin wall portion. The casing contains a vent which ruptures when internal casing pressure exceeds a given value. The vent includes at least one vent-forming rib projecting outwardly from a circular end wall. The rib has formed therein a vent-forming groove which extends transversely along a portion of the length of the rib. Thus, the groove ends are spaced a certain predetermined distance from the base of the rib.
Romero's design has a disadvantage in that the groove does not extend to the rib base on each side, thereby restricting the size of the vent hole that opens when excessive pressure builds up in the casing and a crack propagates in the groove. It is advantageous to have a large vent opening to permit quick release of pressure, and to minimize blockage of the opening due to salts or other impeding particles that may be contained in the container. Romero's reason for having the groove not extend to the bottom of the base is to minimize corrosion (see col. 2, lines 14-31).
Gregory A. Patterson et. al., U.S. Pat. No. 4,610,370, Sept. 9, 1986, discloses a pressure release vent for a container. The container has in a wall thereof an indentation which is interrupted by a scored hollow rib. The rib has an apex which connects the opposite side walls of the indentation at a distance above the bottom of the indentation. This pressure release vent works reasonably well, but it has a number of factors which bear improvement. The opening which is created by the pressure release vent upon fracture is only of pinhole size and hence gas pressure is not expelled quickly. Moreover, since the vent opening is pinhole size, salts or other solid objects in the container tend to jam the opening, thereby impeding the efficient operation of the vent opening. A further problem is that while the pressure release vent is intended to release at pressures of about 450 psig, it is difficult to maintain a high degree of quality control. Unless high quality dies are used, consistent quality control is vulnerable to die wear. In some cases the vent may not release until pressures of 750 psig are reached.
J. A. Oswald, U.S. Pat. No. 4,789,608, Dec. 6, 1988, discloses a pressure venting device for a battery casing which includes two semi-circular concavities extending upwardly from the bottom surface of the casing. Two oppositely disposed bridges interrupt the concavities. Two score lines are disposed laterally and offset inboard from the bridges. Oswald alleges that since the scores are formed in a flat area of the bottom surface of the battery casing and are not affected by subsequent rib stamping procedures, venting will occur consistently at a predetermined pressure range. Oswald emphasizes that quality control is an important objective in forming battery casing pressure venting devices which have a release point within a consistent relatively narrow range.